Knowledge of the rotor position is essential in order to commutate the phase windings of a brushless motor at the correct times. A permanent-magnet motor will often include a Hall-effect sensor, which outputs a signal indicative of the rotor position. Although the component cost of the sensor is relatively cheap, integrating the sensor within the motor often complicates the design and manufacture of the motor. Additionally, the signal output by the sensor is often susceptible to electromagnetic noise generated within the motor.
Sensorless schemes for determining indirectly the position of a rotor are known. For a permanent-magnet motor, transitions in the polarity of the back EMF induced in a phase winding may be used to determine the rotor position. However, since the magnitude of the back EMF is proportional to the speed of the rotor, transitions in the polarity of the back EMF cannot always be reliably determined at low speeds.